1997A&AS..124...23P

With asteroseismic techniques it is possible to investigate the interior and the evolutionary status of stars via their frequency spectrum. Both information would be very much needed for λ Bootis stars, a group of metal-poor Population I, A-type stars, since no conclusive theory exists explaining the observed abundance anomalies. Geneva and Stroemgren photometry place these stars inside the classical instability strip or at least very close to it. We therefore have started an extensive photometric survey for pulsation in λ Bootis stars and have discovered so far 13 new variables. In this paper we present results for stars which presumably are constant, because we are able to establish only an upper level for possible variability. A typical noise level of 3mmag for Stroemgren b was achieved in the relevant frequency domain up to 100d^–1. Considering the given noise level of our survey, we conclude that at least 50% of all investigated λ Bootis stars inside the instability strip are pulsating, making this group remarkable compared to stars with similar spectral types. This may suggest that a low (surface) metallicity has an influence on the pulsation behavior of stars. New models, as well as further observations are needed to clarify the λ Bootis phenomenon using asteroseismic techniques.